Nigeria's new president-elect, Bola Tinubu, called on citizens to unite around him on Wednesday, as he defended the integrity of the national election he won amid a bitter dispute over the results.

Both of the two main opposition parties have rejected the outcome as fraudulent, and said they would challenge the results in court.

The bitter dispute has raised fears of violence in Africa's most populous nation and leading energy producer, which has a long history of electoral violence. In past polls, street gangs with loyalty to Tinubu in the commercial hub of Lagos have fought pitched battles with gangs loyal to rival parties.

"I am very happy I have been elected the president of the federal republic of Nigeria," Tinubu said to cheers in Abuja. "This is a serious mandate. I hereby accept it."

He now faces a litany of national problems, including Islamist insurgencies in the northeast, armed attacks, killings and kidnappings, conflict between livestock herders and farmers, cash, fuel and power shortages, and deeply entrenched corruption.

The Independent National Electoral Commission (Inec) said Tinubu garnered 37 percent, or 8.79 million votes, in the weekend election, ahead of main opposition challenger Atiku Abubakar's 29 percent, or 6.98 million votes.

Peter Obi, an outsider popular with younger and more educated urban voters, won 25 percent, or 6.1 million votes.

With total votes cast at just under 25 million out of 87 million people with voter identity cards and eligible to vote, turnout was only 29 percent, low even by Nigerian standards.

The 2019 election saw 35 percent turnout.

The electoral process encountered problems, owing to new technology that did not function well and seemed to overwhelm Nigeria's notoriously inadequate communications network. That undermined trust in the whole process.

"In the eyes of God, the man (Tinubu) is not the winner," trader Mercy Efong said in Akwa, in Obi's home state of Anambra.

The Inec had promised to upload results from each polling unit to its website in real time, but most units were unable to do so immediately. That was not a legal requirement, but it meant results had to be collated manually inside ward and local government counting centres as in previous polls, reneging on a policy that was meant to improve transparency.

"President Buhari said that he would do free and fair election (but) Inec is now turning everything upside down," said rickshaw driver Nedu Chukwunata, referring to the outgoing president, Muhammadu Buhari.

Observer missions have criticised the problems as the result of poor planning. (Reuters)